i own a 1990 RS Turbo and iam having problems with the speedo, it will be fine for awhile then it just drops to zero and the fuel computer goes to zero and the mileage will not clock up, i thought it was the bearings in the gearbox losing the drive to the speedo but i change the clutch and had the gearbox checked out and its fine. The speedo cable looks ok and the sender for the fuel computer is fine. is it poss to be the clock being faulty if so why does my fuel computer go at the same time.

cheers for any advise